NuclearReactor 1.1 - How does a neutron reflector work?
The neutron reflector is the surrounding space of the reactor core filled with moderator. Nothing else. Very often, the moderator is water, but there are types of reactors using graphite or heavy water. The moderator brakes the fast neutrons released during the fission act. This happens by an elastic scattering. Similar to what we observe when playing pool billiard, the neutron hits the nucleus of the moderator substance, e.g. a proton being the nucleus of hydrogen in the water, and it transfers its momentum to the moderator nucleus. Translated to pool billiard, the first ball hits the second one and stops while the second starts moving with the previous velocity of the first one. Elastic scattering is not always so optimal, but the neutrons always loose energy and become slower. In the end, the neutrons obtain a comparatively low speed (2200 km/s), which is equal to the thermal movement of the moderator atoms. This is the reason why we speak about "thermal" neutrons. The purpose of moderation is to improve the chance of the neutrons to trigger new fission acts. Slow neutrons are much better in this business than fast ones. In this way, the chain reaction can be sustained much easier than without moderator.
But what does the moderator do outside the reactor core? Pretty easy: All neutrons leaving the core are lost for the chain reaction unless something reflects them back into the core. You can see this after starting the programme. In this state, the surrounding space of the core is still empty, which is shown with the white colour. Start up the reactor and try to stabilize the power. Find a control rod position where the power doesn't change after extracting the neutron source. Now the reacotr is critical. How to do this, you can read in the main manual.
Now switch on the reflector by clicking on the button "reflector". The space around the core fills with yellow = moderator. Immediately fast neutrons start to be moderated by the reflector. Some of them manage to migrate back into the core, which also happens with some of the thermal neutrons that left the core. The result: The reactor becomes supercritical without changing the control rod position, indicated by a rise of the power. This kind of reflection is not working like a mirror, it is rather a diffuse reflection. For this reason, the back-scattered neutrons are called "albedo". You can press the button "reflector" for a second time, then the reflector is again taken out and the accumulated neutrons disappear. Don't hesitate to experiment with all the control elements - the worse that can happen is a scram = emergency shut-down.
There is still a second button that is new in version 1.1 of NuclearReactor. It's the "Drain Moderator" button. When you press this, the moderator is taken also out of the core itself. The consequence: The chain reaction is stopped very efficiently without the control rods. To drain the moderator is therefore a possible way to perform an emergency shut-down ....but only for small research reacotrs, where the moderator is not used as a coolant in the same time! In a power reactor, often water is moderator and coolant in the same time. The coolant is still needed to remove the residual decay heat from the fuel that is continuously produced by the radioactive fission products even if no fission is happening anymore. The corresponding thermal power can amount up to 6 % of the nominal power of the reactor. An example of a reactor that disposes of a moderator draining system for fast shut-down is the training reactor of the University in Zittau, the so-called Zittauer Lehr- und Forschungsreaktor (ZLFR).